Let's loot at how to make a thunder effect in Photoshop CS :)
open a new document in size of 400x400 / 72 pixel RGB
Hit the D key first. Then choose the Gradient Tool from the Tool Palatte and fill the document like you see in the left.Now let's do these three steps: 1. Filter>Render> Difference Clouds 2. Image>Adjustments> Equalize 3.Image>Adjustments> Invert
Our picture should look like the right side. Last step we will give a color.
Apple+L or Image>Adjustments>Levels...
with these values.
Apple+B or Images>Adjustments>Color Balance...
arrange the little triangles shown below.